1. Field of the Invention
The present invention is related to a switch controller, and more particularly, to a switch controller for switching power supply.
2. Description of the Prior Art
Applied in AC/DC converting switch power supplies, a common technique to detect the electrical energy held in the primary winding is by utilizing an auxiliary winding. By detecting the voltage across the auxiliary winding, the switch controller is able to determine the timing to turn on or turn off the power switch. For instances, when the switching power supply is utilized for power factor correction (PFC), one operation method is to provide a fixed on-time, i.e. fixed turned on time of power switch, for the power switch. After the power switch is off and the electrical energy held in the primary winding is depleted, the voltage of the auxiliary winding starts oscillating such that the voltage level drops; after the voltage level across the auxiliary winding is detected to be lowered to a certain degree, the current flow in the primary winding can then be deemed as zero and the power switch is turned on again so as to enter the next switch cycle.
However, since the auxiliary winding is coupled to the switch controller, when the voltage level across the auxiliary winding is negative, the switch controller may receive the negative voltage directly, resulting in the latch-up phenomenon and thus making the switch controller operate improperly.